Into a fluidized bed in a combustor, for example an FBC or a PFBC power plant, there is fed particulate fuel in a dry state via a large number of feed nozzles. The fuel may also be fed in by adding liquid, whereby a paste is obtained. Paste feeding is not discussed in this specification.
A large number of nozzles is justified by the need to distribute the fuel in the bed as efficiently as possible thereby achieve complete combustion of the fuel.
Usually, the fuel is fed from a storage container to the nozzles by means of mechanical and/or pneumatic conveyors, which entails a number of parallel rotary vane feeders, or other devices, for controlling the fuel flow to the nozzles.
Rotary vane feeders, or other feeding devices, are expensive, so the aim is to reduce the need of such devices. This may mean that two parallel rotary vane feeders may, for example, be arranged to feed two separate systems of supply conduits with a network of feed nozzles associated with each such system. A natural disadvantage of such a system with two feeding systems which by means of, for example, rotary vane feeders feed fuel more or less independently of each other is the difficulty of being able to adjust the quantity of fuel individually for each nozzle in the fluidized bed. To desire individual control of each nozzle may appear to be contrary to what has been said above regarding the importance of achieving an even distribution of a fuel fed into the bed. The main principle, however, is to attain the good distribution of the fuel. However various factors may contribute to the desire to control the flow of fuel fed into the bed individually for each nozzle. Such factors may be the vicinity to combustor walls and heat-absorbing tube surfaces, differences in mixing and fluidization patterns at or near the nozzle, etc.
U.S. Pat. No. 4,655,147 discloses a method of achieving a simplified pneumatic feeding system of particulate fuel to a fluidized bed. This is achieved by arranging a fuel distributor downstream of a flow-controlling feeding device. The aim of this fuel distributor, placed near the combustor, is to reduce the number of supply conduits between the flow-controlling feeding device and nozzles in the bed. Also, a device according to this prior art suffers from the disadvantage that the flow of fuel cannot be controlled separately from each nozzle.
It is known in the prior art to feed the particulate fuel to the fuel flow distributor in the form of a separate vessel, so that fuel is fluidized to a bed by supplying fluidizing air via a perforated bed bottom, on which the bed of fuel rests. Above the surface of the fuel bed, there is a freeboard. At a bed surface, a number of discharge pipes from the fuel flow distributor are arranged. These discharge pipes lead to the fluidized bed in a combustor. These pipes then serve as feed pipes to the bed of the combustor, the feed pipes being provided with nozzles at their orifices into the fluidized bed. The principle of such a described fuel flow distributor is that fuel in fluidized state, when the bed surface reaches the discharge pipe, will be fed out of the fuel flow distributor and thus be fed into the combustor bed at these discharge pipes. Also in this case, it is not possible to control the feed quantity for each nozzle in the bed individually.
The object of the invention is to feed and control, by means of a fluidized fuel flow distributor, and in an inexpensive manner, the distribution of fuel to a large number of feed points in a fluidized bed, i.e., a so-called FBC, for combustion of the fuel.